In Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) standard, a packet data convergence protocol layer is only applied to a Packet Service (PS) domain, a Radio Access Bearer (RAB) of each PS domain is associated with a Radio Bearer (RB), each RB is associated with a Packet Data Convergence Protocol (PDCP) entity, and each PDCP entity is associated with a Radio Link Control (RLC) layer entity. Each PDCP entity may have many different types of head compression protocol. At User Equipment (UE) side, the PDCP entity provides service for the Non-Access Stratum (NAS) and realizes signal relaying function in a Radio Network Controller (RNC).
The service provided for the upper level by the PDCP entity is called radio bearer; the main functions of the PDCP layer include: (1) respectively completing the head compression and decompression for the IP data stream in the transmitting and receiving PDCP entity, for example, completing the head compression and decompression for signal of the TCP/IP and RTP/UDP/IP of the IPv4 and IPv6; (2) transmitting the user data; and (3) maintaining the serial number of PDCP entity for the RB which is configured to support the relocation of lossless Serving Radio Network Subsystem (SRNS).
In the cellular mobile communication system, because the coverage of single base station is limited, when the UE enters another cell from one cell during the communication process, in order to ensure the continuity of communication, the system transfers the connection between the UE and original cell to a new cell, which is cell handoff. The UE can perform handoff among different cells which are under the same base station controller as well as among different systems and different base station controllers. When the UE switches the base station controller during handoff, one needs to relocate the UE. In the TD-SCDMA system, the process of handoff from the source RNC to the target RNC causes the relocation of SRNS.
To support the lossless relocation function, the PDCP entity is added with a Serial Number (SN) window; according to description of the 3GPP TS 25.323 protocol, the PDCP entity in an SN window is what the Protocol Data Unit (PDU) needs to transmit to the RLC layer entity; and the 3GPP TS 25.323 protocol clearly prescribes that there are unconfirmed PDU records stored in the SN window, and the maximum number of the stored unconfirmed PDU records is the size of the configured SN window, that is, 255 or 65535. After the service of PS domain starts, the data size is large, and more exchanged data packets are needed for the upper-layer application; in addition, when the PDCP entity supports the lossless relocation, after receiving a confirmation of PDU from the RLC layer entity of the opposite end, the RLC layer entity at this side reports the confirmation of PDU to the PDCP entity; however, due to the bad signal of network transmission and other reasons, the PDU just fed back and confirmed by the RLC layer entity of the opposite end is not necessarily the PDU associated with the first PDU record stored in the PDCP entity at this side, so, in the SN window of the PDCP entity, a PDU record of a confirmed PDU cannot be deleted immediately, but it is needed to determine whether the PDU of a previous PDU record is confirmed; the PDU record of the confirmed PDU can be deleted only after the PDU of the first PDU record is confirmed.
Thus, if the PDU of the first PDU record is not confirmed, the subsequent PDU records are piled in the SN window, until the SN window is jammed.
Currently, there are two processing methods as follows: a simple discarding method and an earliest discarding method.
The simple discarding method, as shown in FIG. 1, has the following steps:
Step 101: after receiving a Service Data Unit (SDU), the PDCP entity reads the number of PDU records in the SN window, which is defined as W;
Step 102: the PDCP entity determines whether the W is less than an initially configured size of the SN window, that is, 255 or 65535, if so, the PDCP entity executes Step 103, otherwise, the PDCP entity executes Step 104;
Step 103: the PDCP entity encapsulates the SDU into a PDU in accordance with the 3GPP TS 25.323 protocol and transmits to the RLC layer entity, and adds a PDU record of this PDU into the SN window; and
Step 104: the PDCP entity discards the SDU.
The earliest discarding method, as shown in FIG. 2, has the following steps:
Step 201: after receiving an SDU, the PDCP entity reads the number of PDU records in the SN window, which is defined as W;
Step 202: the PDCP entity determines whether the W is less than an initially configured size of the SN window, that is, 255 or 65535, if so, the PDCP entity executes Step 203, otherwise, the PDCP entity executes Step 204;
Step 203: the PDCP entity encapsulates the SDU into a PDU in accordance with the 3GPP TS 25.323 protocol and transmits to the RLC layer entity, and adds a PDU record of this PDU into the SN window; and
Step 204: the PDCP entity discards the first PDU record in the SN window, encapsulates the SDU into a PDU in accordance with the 3GPP TS 25.323 protocol and transmits to the RLC layer entity, and adds a PDU record of this PDU into the SN window.
The simple discarding method above, when the SN window is full, discards the received SDU directly, which is problematic for not solving problem essentially, namely, not considering the reason why the SN window is full, thus causing the loss of SDU once the SN window is full; the earliest discarding method above, when the SN window is full, discards the earliest PDU record in the SN window so as to convert the newly added SDU into a PDU to be transmitted to the RLC layer entity, which is problematic for not trying to store the unconfirmed PDU record for a time period as long as possible, thus making it impossible to retransmit the PDU needed by the opposite end in case of the relocation of a lossless SRNS.